It is well known that when polyesters are exposed to high temperatures, as they often must be during processing and application from the molten state, they tend to degrade with detrimental loss of physical and chemical properties such as a decrease in molecular weight and discoloration. This condition often results in inferior products at best, or worse, it renders these polyesters useless in the intended application. Therefore, it would be desirable to have polyester compositions with improved high temperature and high humidity stability.
Organic salts of alkali metals have been combined with polyethylene terephthalate type polyesters. U.S. Pat. Nos. 5,017,680 and 3,962,189 disclose catalyst-inhibitor systems for preparing polyethylene terephthalate with a fast reaction rate and good color. U.S. Pat. No. 5,106,944 discloses a catalyst inhibitor system for preparing poly(1,4-cyclohexenedimethylene terephthalate) with a fast reaction rate and good color. The catalyst inhibitor system of U.S. Pat. No. 5,017,680 contains organic salts of alkali metals along with a complex of titanium alkoxide. The catalyst inhibitor system of U.S. Pat. No. 3,962,189 contains organic salts of alkali metals along with manganese, zinc or calcium, antimony, cobalt, phosphorous, and titanium. The catalyst inhibitor system of U.S. Pat. No. 5,106,944 contains organic salts of alkali metals along with a complex of titanium alkoxide. The patents do not include poly(ethylene-2,6-naphthalene dicarboxylate), and do not address hydrolytic stability.
U.S. Pat. No. 4,357,461 discloses a process for preparing a heat stable polyester resin wherein an alkali metal salt of ethylenediaminetetraacetic acid (EDTA) is present in the melt polymerization reaction mixture. The patent does not include poly(ethylene-2,6-naphthalene dicarboxylate), and is chiefly concerned with reducing the acetaldehyde generation rate.
In contrast, the present inventors have unexpectedly determined that the hydrolytic stability of poly(ethylene-2,6-naphthalene dicarboxylate), referred to as PEN, particularly at high humidity (&gt;50% relative humidity) and elevated temperatures (&gt;23.degree. C.), can be significantly enhanced by the addition of 15 ppm to 100 ppm of an alkali metal derived from an alkali metal salt to the melt polymerization reaction mixture.